1. Technical field of the invention
This invention relates to gas depolarized electrochemical batteries comprising at least one cell, particularly those consuming oxygen from the air. An efficient semiconductor microactuator (a valve-on-a-chip) is placed on a sealed battery (a battery including at least one cell) so that the semiconductor microactuator is the sole means of entry of fluid depolarizer, most often air, to the battery, permitting the battery to operate when the battery is supplying electrical current to a load. The invention also encompasses micromachining a valve mechanism. The invention excludes fluid depolarizer and impurities when the battery is not supplying electrical current to an electrical load to prevent the battery from discharging and losing power capacity while not in use. The semiconductor microactuator will break down in such a way when the battery "leaks" to minimize damage to the device the battery is operating. The semiconductor microactuator acts as a pressure relief valve. The semiconductor microactuator may also be designed to act more optimally as a safety pressure valve or as a fuse. The battery is rechargeable and this invention covers the combination with a recharger and with a control device to maximize the charge.
2. Background and Description of Related Art
Gas depolarized cells exist in many types and varieties. The most common in commercial use today are metal-air cells, especially zinc-air cells. In a zinc-air cell, the oxygen in air, by a series of reactions, reacts with the zinc in the cell, producing electrical current. Most of the variety of gas depolarized cells to which this invention relates are described in McArthur et al., U.S. Pat. No. 4,547,438, Oct. 15, 1985, Zupancic, U.S. Pat. No. 4,529,673, Jul. 16, 1985, Mathews et al, U.S. Pat. No. 4,177,327, and literature cited in those patents.
The principal advantage of zinc-air cells is that higher energy density--, i.e., meaning watts per unit of mass, can be achieved using oxygen of the air, or other gas, as a "fluid" cathode material. This is, instead of, for instance, the solid material found in a typical home flashlight battery. A cell of a given standard size can contain much more anode and electrolyte volume because the oxygen reactant is "stored" outside in the atmosphere. This is useful in small devices such as hearing aids, and also useful in larger cells, such as flashlight "D" or "C" cells, or in the largest of batteries such as in an electric car where much power is needed, but space used takes away from space for other uses. Similarly, a cellular or portable phone is a good use.
Further describing the general design and technical aspects of the cell, in a typical cell (or combination of cells, referred to as a battery) of this type, the negative side of the cell, or anode, is made of a metal, most often zinc, and less often of aluminum and magnesium because of their high electromotive potential. The positive side, or cathode, is usually manufactured as a plastic-bonded, metal-oxide-and-carbon-porous body adhered to a conductive screen or layer. The metal oxide acts as a catalyst for oxygen reduction and to give the cell start-up power as gas containing oxygen, usually air, diffuses into the cathode pores when the cell is activated. The electrolyte is the material in the cell through which charged ions may pass and where key steps of the chemical reaction producing electrical potential or voltage occur. Schumm, Jr., "Batteries," Encyclopedia of Physical Science and Technology, vol 2, p. 387, 390, 396-97 (Academic Press, Inc. 1987). The metals which can be used include, for example, lead, calcium, beryllium, and lithium and alloys and mixtures of those elements. The use of the word "air" is employed for convenience to mean an oxygen source, which source could thus be other gas mixtures including oxygen.
In a single gas depolarized electrochemical cell, for instance one of the Gould type, Cretzmeyer et al., U.S. Pat. No. 4,189,526, the air enters through vent holes in the outside container of the cell through a polytetrafluoroethylene (often sold as "Teflon", Trademark of Dupont Co.) layer.
When such a cell is not operating, the reactant fluid, oxygen in the air, as well as other impurities, must be excluded. Previously, no combination of a valve and battery existed where the parasitic use of power by the valve did not substantially diminish the life or the power of the cell or consume too much space or structure.
Excluding fluids and depolarizing gas prevents the cell from degrading through several processes of corrosion, moisture change and impurity entry which: a) shorten the "shelf" or storage life of the cell when it is not in use, and b) necessitate more frequent changes of the cell in an electrically powered device. Since a common use for this type of cell is for a hearing aid, it is commercially useful not to have to change the battery so frequently. Another common use for the cell is in a buoy at sea; exclusion of the humid, salty sea air when the cell is not operating and reduction of the frequency in changing the cell, or cells in a battery, save much labor and money. The control of the passage of water vapor by the valve prevents the cell from swelling or otherwise being damaged, and prevents dehydration of the cell while not operating. Also, carbon dioxide, which degrades the performance of the cell, is precluded from entering the cell when the cell is not operating.
Previous engineering designs used a variety of means to attempt to overcome these problems. Several inventions used a mechanism physically operated by the user where the valve or vent cover is attached to the switch turning a device "on" so that when the switch moves, the cover moves. Derksen, U.S. Pat. No. 2,468,430 dated Apr. 26, 1949; Cheiky, U.S. Pat. No. 4,914,983 dated Apr. 3, 1990, and H. R. Espig and D. F. Porter, Power Sources 4: Research and Development in Non-Mechanical Electrical Power Sources, Proceedings of the 8th International Symposium held at Brighton, September 1972 (Oriel Press ) at p. 342. The physical presence of the operator is required, as well as a device designed with a switch compatible with the battery system.
Another obvious and long-known approach is a solenoid or electromagnetic means to move a valve or cover as the device is turned on or off, which, consumes a substantial amount of the power of the cell or takes up substantial space.
A more primitive approach which is effective before the cell is operated is to place a sealing tab or plug on the cell (like a pull-tab on a soda can) to be removed when the cell is put in service, admitting oxygen to the assembly. The sealing tab or plug in combination with an airtight assembly at least prevents the deactivation of the zinc from external sources before use (while on the "shelf"), but once activated by removing the tab, small cells must be used completely within 1 to 3 months, or the cell will have self-discharged or dried out with no useful power remaining. If the cell is operated continuously, this "once-opened/always-opened characteristic makes little difference, but since most electrical devices are at least occasionally turned off for a period of time, a recloseable valve is important to protect the cell from degradation during that time.
The art of Mathews, U.S. Pat. No. 4,177,327, Dec. 4, 1979, previously mentioned, contemplates using a vent cover, in the form of a plug or a flap, in conjunction with an electrical heating element. The bimetal element in the Mathews patent is referenced as 1.625 inches (4.1 centimeters) long. The heating element was referenced to cover 0.75 inches (1.9 centimeters). The embodiment in the Mathews patent contemplated that the bimetal element would move to produce a clearance of 0.30 inches (0.75 centimeters). The moving portion of the vent cover assembly in the Mathews invention is parallel or roughly parallel to the flow of air into the cell. That position requires either 1) a significant loss of dimension in the length of the cell, if the cell is a cylinder, 2) a reduction in the available space of a cylindrical cell by creation of a cavity in the side of the cell or 3) a reduction in the height of the cell to accommodate the vent cover and heating apparatus. By comparison, a typical hearing aid battery is 1.16 centimeters ("cm.") in diameter and 0.42 cm. to 0.54 cm. thick, a typical "C" size flashlight battery is 2.6 cm. in diameter and 5 cm. high; a typical "D" size flashlight battery is 3.4 cm. in diameter and 6.0 centimeters high (The valve-on-a-chip is 0.4 cm..times.0.4 cm..times.0.1 cm. in total size).
The present invention uses significantly less space and is therefore suited to a single small cell configuration and avoids the loss in energy density either because of lower power drain or less space consumed or both.
In addition, the present invention is intended to be used, which was not claimed or disclosed in the prior art, to act as a pressure relief valve because the cover is to the exterior of the inlet to the interior of the cell or battery so that a plug or flap is not "trapped" against the outside container of the cell or battery. Further, another objective not disclosed or intended in the prior art, is to use corrosive fluid when the battery "leaks" to clog or to distort the semiconductor microactuator and causes the cell to cease to function generally by oxygen deprivation, although it may also occur by damaging the heating element which opens the semiconductor microactuator. This more reliably causes the battery to cease to operate when it is leaking than in prior art.
In all, the difficulty has been to produce a combination that preserves the energy density of the cell and at the same time provides a cell that can be "dropped into" a device and function automatically to preclude fluid and impurity entry while the cell and device are not operating. In addition, a pressure relief characteristic and "shutdown" of the cell on malfunction or "leakage" would be helpful, but all of these functions together cannot be achieved in the prior art.
Previously, limited efforts had been made to have certain types of valves on liquid electrolyte electrochemical cells as well. The prior art of Cheron, U.S. Pat. No. 4,039,728, is related to a valve which consumed substantial power which was the means, in combination with a fuel cell--, i.e., a special type of fluid depolarized electrochemical cell, to control the circulation of liquid electrolyte in the cell, which valve was actuated based on a parameter which is function of the circulation of electrolyte in the cell.
This invention overcomes the power and space requirements by using the new combination of an automatic valve of different materials and size, preferably a small electronic semiconductor microactuator, a "valve-on-a-chip", after the art of H. Jerman, U.S. Pat. No. 5,069,419, Dec. 3, 1991, (that valve and like valves, including those referenced in that patent, referred to as a "semiconductor actuator valve" or "valve-on-a-chip") in conjunction with a sealed gas depolarized electrochemical cell, especially a zinc-air cell. The sole means of entry of depolarizing fluid is through the valve on a chip. The combination produces a new and commercially useful invention by employing recent advances in semiconductor and micromachining technology that were not previously commercially available or invented. The invention contemplates the use of at least two layers, one of metal and one of semiconductor material, juxtaposed to each other.
The way the valve/cell combination works is that when the electrical device the cell is powering is "turned on", the consequent closing of the operating circuit causes the valve to open, admitting gas, normally air, to the cell. When the circuit is opened, meaning the electrical device the cell is powering is "turned off", the valve closes, precluding entry or exit of fluids or other impurities. The valve does not close as quickly as it opens, but this time is not significant compared to the many hours of time when exposure to the air would be typically closed off, and has the additional advantage of preventing "chatter", or unnecessary vibration, in certain applications.
There is sufficient residual oxygen or oxide compounds in the cell so that the cell will deliver sufficient power to start up and operate the valve-on-a-chip. This can be enhanced by the use of manganese dioxide or other catalytic agents in the cell. Such a cell has a higher starting power before oxygen fully penetrates the cathode pore structure. In any case a potential of generally over one volt exists between the cell electrodes. When the circuit containing the apparatus to be operated is closed, this invention causes electrons to flow from the zinc anode(s) through the electrical circuit and the valve to the cathode(s) of the cell or cells. Tuck, Modern Battery Technology, Ellis Horwood Series in Applied Science and Industrial Technology at generally pp. 126-188 (London, NY 1991).
This valve has the additional advantage that it is conducive to a pressure relief characteristic which continues to be usable as a vent closure after relief of the pressure.